Most conventional compact radio frequency (RF) transceivers are currently built using a plurality of printed circuit boards (PCBs), one for RF circuitry, and another for digital circuitry. In addition, conventional antennae and RF components which are generally physically larger than digital circuitry components do not fit within compact form factor housings for the following reasons.
Although mechanical tuning devices of the older generation transceivers have been eliminated by use of direct conversion (homodyne) receivers with voltage tuned components, together with improvements in digital frequency control such as voltage control oscillators with phase lock loop (PLL) circuits and digitally controlled divider circuits, conventional RF transceivers still include RF components which are too large for incorporation into the newer and more compact form factor housings required by the portable PC environment, such as the PCMCIA form factor. The physical and electrical PCMCIA standards are found in the PCMCIA Standards published by Personal Computer Memory Card International Association, 1030 G. East Duane, Sunnyvale, Calif. 94086. The current version is denominated Release 2, and dated November, 1992. This publication is incorporated herein in its entirety by reference. In addition, most commercially available conventional inexpensive demodulators for use in RF circuits have limited noise immunity. Alternatively, sophisticated demodulators are either too expensive or too bulky for use in the thin form factors required for small portable PC applications.
Small antennas are available for compact RF transceivers, and some smaller antennas have been designed to fit on small PCBs. However, these conventional small antennas are usually not omni-directional, have low radiation efficiency, have poor matching characteristics or have driving impedances that are too sensitive to nearby conducting surfaces, commonly encountered in the PC work environment.
The PC work environment generally comprises multiple PCs and peripherals. As such, it is necessary for an RF transceiver to avoid interfering with another transmission already in progress. If several transceivers are contending for the same channel, the channel is effectively blocked if these transmitters do not have some means of sharing the channel or resolving the contention.
Several conventional interference avoidance techniques have proven to be inadequate. One conventional interference avoidance method uses a carrier detect (CD) system to determine if a channel is occupied. In such a system, the carrier detect circuit causes the transceiver to wait for a clear channel before transmitting. However, in an environment where the allocated frequency range is very narrow, an out-of-band signal or a high noise level environment can easily cause a false "channel-busy" warning from the carrier detector circuitry, thereby preventing operation over an otherwise available channel. This is particularly true in the Industrial, Scientific and Medical (ISM) band where a large number of users employ different types of transmission formats and/or protocols and share the transmission frequency band for unrelated operations. Within the ISM band, there are no defined channels or bandwidth, and no protected channels.
The carrier detection interference avoidance technique is further complicated by the presence of other receiver(s) in the area since their local oscillator (LO) radiations may appear to be carrier signals to the carrier detect circuit. This problem is worse for direct conversion radios where the LO is on the same frequency as the receiver. In addition, other unrelated signals and noise may appear to be traffic on the network. This is because the RF input stage of a conventional direct conversion receiver has a broad bandwidth and therefore any nearby signal source is erroneously interpreted as traffic on the network.
Another conventional interference avoidance technique involves the use of communication command sets which include the attention (AT) command set, which has become the de facto standard (originally specified by Hayes Corporation for "smart modems"). The AT commands were designed to be operated using wired modems and these command sets are not always compatible with the typical operation of RF transceivers which have different contention and hand-shaking problems and hence requirements.
Power consumption is also another major concern in portable transceivers. Most commercially available portable radio transceivers, such as cellular phones, transmit in the 1-3 watt ranges. As such, it is not possible to run such transceivers for an extended period (days) using a small battery, for example, a disposable 9 volt battery or a small number of AAA batteries, commonly used to power the newer generations of palm top PCs. Instead, most conventional portable transceivers require larger Ni-Cad batteries which typically operate for at most 8 hours before requiring a recharge. As such, most conventional RF transceiver designs are unsuitable for incorporation in low power portable computers such as a lap-top or palm-top personal computer (PC).
Sleep modes or low power modes have been implemented in portable RF receivers to extend battery life. These modes must not cause missed messages or impose excessive delay in receiving messages. Conventional methods include using: (a) a low power broad band receiver which remains powered to wake up the main sleeping receiver; or (b) waking a sleeping receiver periodically on a predetermined schedule to listen for expected transmissions. Both of these methods have inherent disadvantages.
A low power broad band receiver will interpret noise or transmissions in a neighboring frequency as a message which should be received and wake the main receiver unnecessarily, thereby wasting power. Alternatively, a timed wake-up type receiver must be synchronized by a central control means and is therefore generally not suitable for random transmissions between independent terminals such as PCs and peripherals.
Yet another conventional method of conserving power involves shutting clocks down and restarting the clocks when needed. However, restarting the clocks and allowing the clock oscillations to stabilize sufficiently before resuming operation of the receiver imposes an undesirable delay in recovery time.
Accordingly, there is a need for a compact, low cost and low power RF transceiver having an efficient contention resolution capability that fits into a housing sized within a compact form factor, for use with PCs and/or peripherals.